Why Does It Make Sense That the Testes Are Actually

Why does it make sense that the testes are actually housed outside the body core?

A) It doesn't. It's a very risky design because they're so sensitive and important.
B) Because male mammals get selected by females based on the size of their testes
C) Because they need more oxygen than would be found inside the body for the sperm to function properly
D) Because they need to be kept at a warmer temperature than normal body temperature in order for the sperm to function properly
E) Because they need to be kept at a cooler temperature than normal body temperature for the sperm to function properly